Method of and apparatus for irradiating matter with high energy electrons



Dec. 12, 1961 J. G. TRUMP 3,013,154

METHOD OF AND APPARATUS FOR IRRADIATING MATTER WITH HIGH ENERGY ELECTRONS Filed Nov. 14, 1958 2| oooooooo United States Patent 3,013 154 METHOD OF AND APPAIRATUS FOR IRRADIAT- lNG MATTER WITH HIGH ENERGY ELECTRONS John G. Trump, Winchester, Mass, assignor to High Voltage Engineering Corporation, Burlington, Mass,

a corporation of Massachusetts Filed Nov. 14, 1958, Ser. No. 773,356 5 Claims. (Cl. 250-495) This invention relates to the irradiation of matter with high energy electrons and in particular to a method of and apparatus for delivering the ionizing energy of a beam of high energy electrons to the matter being irradiated with maximum energy etliciency and minimum side effects due to excess dosage by appropriate focusing of the electron beam through an electron optical lens system. In a co-pending application, Serial No. 519,774, assigned to the assignee of the present application, and which is now US. Patent 2,866,902, granted December 30, 1958, a device is shown wherein an electron beam of circular transverse section is created in an acceleration tube. That electron beam has a non-uniform distribution of charge density and in accordance with that invention non-linear methods of spreading are applied in order to secure a uniform distribution over an extended line. In accordance with the present invention the starting point is a short uniform line source of electrons. The creation of an electron beam having an extended line cross section is more fully disclosed in my co-pending application Serial No. 594,243 (now abandoned). In accordance with the present invention a short uniform line source is provided and then rather simple magnetic fields are employed to spread out this beam, or both to deflect it and then spread it out. As will appear, the present invention not only has advantages over the disclosure of the aforementioned co-pending application, but the present invention also has ad vantages over the rapid spot-scanning which is disclosed and claimed in US. Patents Nos. 2,602,751 and 2,729,748. In the following description not all the possible variations of the present invention are described in detail. For example, the spreading and deflection required and hereinafter described can be done with electric as well as magnetic fields. However, magnetic fields would be much preferred and accordingly in the following disclosure particular reference is made thereto.

High energy electrons are an important form of ioniziug energy and high energy electron sources are finding increasing application in radiation chemistry, sterilization and preservation of food and drugs and similar fields. A high energy electron source may be provided by accelerating electrons to high energy in an evacuated tube and permitting the high energy electrons to issue from the tube through an appropriate electron window onto the matter to be irradiated.

When a high energy electron beam issues from an evacuated acceleration tube into the atmosphere, the electrons are scattered by the gas molecules; hence, although the diameter of the electron beam may be maintained quite small within the vacuum, the diameter of the same beam may be about four or five inches at a distance of about two feet from the electron window. The most natural way in which to irradiate a substance with such an electron beam is to place the substance at about two feet from the electron window where the beam diameter is adequate to cover the surface of most substances which one might wish to irradiate, and in fact this technique has been and still is frequently employed.

However, the transverse distribution of electrons in the beam is normally characterized by a central region of relatively high intensity which attenuates gradually at 3,013,154 Patented Dec. 12, 1961 increasing distances from the axis of the electron beam. This uneven distribution of electron current density means a loss of efficiency since the excess power delivered near the axis of the beam is Wasted. Such efficiency loss is highly objectionable where the power delivered is costly, as is the case in electron irradiation. Moreover, frequently the dose required to obtain a desired result by electron irradiation is only slightly less than a dose which may produce undesired side effects. Thus the overdosage near the beam axis not only reduces efficiency but may even be harmful.

My invention saves the use of ionizing energy which would otherwise be wasted, avoids the damaging effects of excess dosage and provides a source of high energy electrons which can deliver an accurately predetermined dosage which can be adjusted merely by controlling the current and voltage of the electron beam. Although various means of accomplishing this same result have been proposed, such as, forinstance, beam scanning and non-linear beam focusing, my invention has certain advantages over either of these methods, and the essence of my invention consists in the fact that one starts off with an elongated cathode or other means for initially producing an electron beam of elongated transverse section. That is to say, an electron beam is produced which has an elongated, rectangular cross section, as by employing an elongated cathode or a linear string of point cathodes closely spaced. The linear spread of the beam is then increased. The simplest way of doing this is to provide a magnetic field in the path of the beam of more or less uniform field strength, wherein the boundaries of the field are so situated that the electrons have a varying path length of travel through the field. This is a simpler method of spreading the beam than the use of a magnetic field whose intensity varies. In this way the electron beam may be spread out in one direction or in both directions or, alternatively, the electron beam may be deflected through or so and in.

the course of this deflection it can also be spread.

While the invention is particularly useful in connec tion with an elongated electron beam whose current density is substantially uniform along the longer of its cross sectional dimensions, it is clear that the invention is not limited thereto but includes the application of the principles hereinafter disclosed in detail to an electron beam whose current density is predetermined whether or not the current density is uniform. In certain applications it may be desirable that the current density vary somewhat along the longer of the cross sectional dimensions, and the principles of the invention apply equally well to such a case as to the more usual case where a uniform current distribution is desired.

In order that the principle of the invention may be readily understood, I have disclosed several embodiments thereof in the accompanying drawings wherein:

FIG. 1 is a diagram illustrating one embodiment of the invention wherein an electron beam of extended cross section along one dimension is further spread by means of magnetic fields;

FIG. 2 is a view along the line 22 of FIG. 1;

FIG. 3 is a view along the line 3-3 of FIG. 1;

FIG. 4 is a view of a modified line source of electrons suitable for use in the apparatus of FIG. 1;

FIG. 5 is a view along the line 55 of FIG. 4; and

FIG. 6 is a view similar to that of FIG. 1 and showing a modification of the invention in which the electron beam is deflected at an angle and then spread. 1

Referring to the drawings and first to FIGS. 1,2 and 3 thereof, an electron accelerator 1 produces a beam 2' of high energy electrons which is directed onto the matter 3 'to be irradiated.

which are then directed through a series of electrodes at progressively more positive voltages with respect to said line source 4 of electrons. These electrodes 5 have apertures 6 therein for passages of the electron beam 2' and serve not only to accelerate the electrons but also to maintain the line distribution of electrons. The linear spread of the beam 2 is increased after acceleration by the provision of a magnetic field transverse to the plane of said line beam 2 so as to spread it through an angle of at least five degrees on either side of its original direction or on both sides of its original direction as shown in FIG. 1. This magnetic field may be produced by two pairs of permanent magnets 7, 3 having pole faces which form a magnetic gap through which the beam 2 travels. The orientation of the magnetic field produced by one pair 7 is opposite to that produced by the other pair 8. It is also possible to use simple magnetic shims beneath the main magnetic field to correct non-uniformity of the magnetic field or even to introduce it. One such shim is shown at 9 in FIG. 1. The electrons then issue through an electron permeable window 10 of extended length.

As is apparent from the foregoing description, the essence of the invention resides in starting off with a short uniform line source of electrons and then using rather simple magnetic fields to spread the beam out further. The simple line source of electrons may be created by using a single extended filament as shown in FIGS. 1, 2 and 3, or alternatively it may be created by a rectilinear series of point cathodes 11 as shown in FIG. 4.

An alternative construction embodying the invention is shown in FIG. 6. Referring thereto, the magnetic field therein indicated not only bends the original beam 2 at an angle (such as 90), but also spreads the linear distribution of electrons over a substantial angle (such as 10 to 90). As is apparent from FIG. 6, these objectives are accomplished by so shaping the boundaries 12, 13 of the magnet pole faces that certain portions of the electron beam 2 travel a longer path through the magnetic field than others, the magnetic field strength being substantially uniform.

As previously pointed out, one ideal shape of transverse section of the electron beam is a true line (as shown at 2 in FIG. 3) and is produced by a linear filament (as shown at 4 in FIG. 1). Another nearly ideal transverse shape is a series of beams of circular cross section arranged in a line (as shown at 2' in FIGS. 4 and 5). Such a beam would be produced by a rectilinear series of point cathodes (as shown at 11) in FIG. 4.

The family of ideas above described on the spreading of a beam of electrons, which may be either steady or pulsed, starts off with (a) a beam which is already a line source. That is to say, one of its transverse dimensions is at least several times and preferably more than ten times the other. This line, for example, might be one-half inch or one inch in its major transvcres dimension. In addition (b) the current density along this line is nearly uniform. Such a beam is then accelerated to high energy using electrostatic lenses which, in certain embodiments of the invention, do not disturb this distribution too much. In other embodiments of the invention this accelerating system can also be designed further to accentuate the major transverse dimension somewhat. I have found that this can be accomplished by an adjustment of the ratio of the voltage of the first group of accelerating electrodes to that of the next adjacent and subsequent groups, although other ways of accomplishing this result could be devised; such an adjustment of voltage ratio can be made by appropriate selection of values for the interelectrode resistances 14 (FIG. 1). After acceleration the line beam passes through a magnetic field adapted to spread the beam over the desired angle. If desired, this magnetic field may also deflect the beam through any desired angle (as shown in FIG. 6). The beam having been spread travels on until its major transverse dimension has reached the desired size, and

preferably then passes from vacuum through an electron window into the world.

This invention has several advantages over the rapidspot scan disclosed in US. Patents Nos. 2,602,751 and 2,729,847. In the first place the electrons are at any instant passing through all of the window with subsequent lower local heating and avoidance of the alternations of hot and cold at any point. This would certainly be important at very high output power. Secondly, the spreading is done with a stationary and constant magnetic field for a given voltage. Thus it can be done with permanent magnets. Thirdly, scanning frequency limitations are avoided. Fourthly, this invention is applicable to pulsed microwave or AC. accelerators as well as DC accelerators. As a general rule, application of the invention to AC. accelerators would require the use of electromagnets with a flux density adjusted to voltage variation. However, in the case of A.C. accelerators which use only a region near the peak of the voltage waveform, a compromise steady magnetic field could be found.

My invention may conveniently be used with an electron accelerator having a space charge limited cathode. Use of such a cathode eliminates difficulties which might otherwise be experienced in getting a repeatable nearly uniform line source of electrons. In addition, it is desirable to compensate the change in both angular spread and beam deflection which may be produced by movement of the beam in the plane of the beam and its long trans verse dimension.

Having thus described the method of my invention together with several illustrative embodiments of apparatus for carrying out the method, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth by the following claims.

I claim:

1. Apparatus for delivering the ionizing energy of a beam of high energy electrons to matter to be irradiated with a predetermined distribution of dosage over its area. comprising in combination: an evacuated chamber; a line source of electrons within said evacuated chamber having predetermined electron emitting properties and having transverse dimensions such that one is at least several times the other; a system of electrodes adapted to accelerate said beam within said evacuated chamber to high energy; electron optical means for imparting angular divergence to the accelerated beam; and an electron window in the wall of said evacuated chamber spaced from said electron optical means a sufiicient distance so that said beam is substantially spread out as it travels therethrough.

2. Apparatus in accordance with claim 1 wherein said electron optical means includes at least one magnet forming at least one magnetic gap through which said beam passes.

3. Apparatus in accordance with claim 1 wherein said electron optical means includes magnets forming at least two magnetic gaps in which the magnetic fields are oppositely oriented for spreading out this beam, and at least one magnetic shim adapted to produce a corrective magnetic field in the vicinity of the junction of said oppositely oriented magnetic fields.

4. Apparatus in accordance with claim 1 wherein one of the transverse dimensions of said line source of electrons is at least ten times that of the other.

5. Apparatus for delivering the ionizing energy of a. beam of high energy electrons to matter to be irradiated with a predetermined distribution of dosage over its area comprising in combination: an evacuated chamber; a line source of electrons within said evacuated chamber having predetermined electron emitting properties and having transverse dimensions such that one is at least several times the other; a system of electrodes adapted to accelerate said beam within said evacuated chamber to high References Cited in the file of this patent UNITED STATES PATENTS 2,602,751 Robinson July 8, 1952 5 2,866,902 Nygard Dec. 30, 1958 FOREIGN PATENTS 764,337 Great Britain Dec. 28, 1956 

